The Patagonian cavy is shaped like a long-legged rabbit or hare. The hind legs are slightly larger than the front. The front feet have 4 toes and the back feet have 3 toes. All toes have a sharp claw. They have large eyes and a large nose with slightly pointed ears. They are grayish-brown dorsally and white underneath with a large white patch on their hindquarters. Head to tail length is 27 to 31 inches (70 to 80 cm); with a maximum tail length of 2 inches (5 cm). Adults weigh 17 to 40 lbs. (8 to 18 kg).

Female cavies are sexually mature at 3 months and males at 6 months. Each pair is monogamous for life, breeding 2 to 3 times each year around midwinter and spring. Gestation is 3 months and each litter has 1 to 3 young. The young are well developed, with open eyes and can move around soon after birth. At that time, young are moved into a communal den with the young of up to 15 other breeding pairs. The adults return to the den to feed their young. Males drive off other parents to prevent them from nursing their young. Females locate their young by scent and drive off other young. The young are weaned after 2 to 3 months.

Cavies travel in mated pairs, with the male always following the female as a guard against other males as well as predators. They are not territorial but there appears to be a dominance hierarchy among males. They can be observed traveling in very large groups of up to 70 cavies to places where food is plentiful. They have a variety of gaits: walking, hopping like a rabbit, galloping or stotting (bouncing on all fours). They have been clocked at up to 45 km/hr. They also have a variety of vocalizations, from grunts and grumbles to screams.